DHM Race-Spec High Compression Stock Bore Piston Kit – Honda Grom & Monkey
The highest compression stock bore piston on the market — and the only one with this dome design, made exclusively for DHMotoring. After 6 years of racetrack testing and over a dozen stock bore pistons evaluated, DHM engineered this Gen 2 piston to push the absolute limits of pump fuel compression while maximizing lubrication and long-term reliability at high RPM.
No other stock bore piston has produced more HP on the Honda Grom. If you’re building the most competitive 125cc engine possible, this is your foundation.
Specifications
- ~13.2:1 compression ratio
- 52.4mm stock bore diameter
- Larger ring and skirt oiling holes than competing pistons
- Larger oil spring for increased oil volume between oiling rings
- Oil ports under pin holes to keep the wrist pin lubricated at all RPM
- 360° skirt design for increased overall strength
- Valve reliefs sized for valves up to 27.5mm / 23.5mm — compatible with the highest lift cams on the market
What’s Included
- DHM Race-Spec high compression piston
- Racing piston ring kit with pins and circlips
- New base gasket and head gasket
- Optional: NGK CR9E (6263) spark plug
Requirements
- Minimum 91 octane fuel required
- NGK CR9E (6263) spark plug required
- Cylinder NOT included — piston kit only
- Ring gaps must be set before installation (see instructions below)
Fitment
- Honda Grom OG (2014–2015)
- Honda Grom SF (2017–2020)
- Honda Monkey 1st Gen (2018–2021)
- Honda Super Cub (compatible)
Installation Notes
Installing this piston requires setting the ring gaps. If you are not comfortable with this process, have your local shop perform the install. Failure to set proper ring end gap can result in premature failure or engine seizure.
Recommended end gaps: .007–.009″ on the top ring — .009–.011″ on the bottom ring. The silver ring goes on top, the black ring below. Watch the install video below for a full walkthrough.
